So, a random tweet made me think of Robert A. Heinlein's LORD OF THE RINGS, which in turn put me in mind of how classic fantasy is usually very conservative (in the little c sense). Science fiction writers usually have their own brand of crazy political ideology, but at least few of them share the same idiosyncrasies.
So without further ado, here are synopses of how sci-fi writers would have handled some classics of fantasy:
MOUNT DOOM IS A HARSH MISTRESS by Robert A. Heinlein - Clever, plucky adventurer overcomes monolithic state regulation and transformation into giant floating eye to become successful vulture capitalist. When elven and human monopolists try to break up his free-steading organization, he expands his operations and introduces the Industrial Revolution to his land.
I, HOUSE-ELF by Isaac Asimov - When a house-elf by the name of Dobby breaks the 3 Fundamental Rules of Hogwarts, an investigator from the Ministry of Magic has to figure out what caused this. He discovers a man named Dumbledore is trying to create a cult that will preserve magical knowledge after the collapse of wizard society, hidden as a religion built around a figure called "Harry Potter".
THE DOMINATION OF THE WHITE QUEEN by S.M. Stirling - Sexually-liberated pagan woman flees a disintegrating world to found a new one colonized by survivalists, soldiers of fortune and hunters. She subdues the primitive natives with her superior organization skills and weaponry. By her efforts, she slows global warming and does away with a hopelessly repressed, sexist society. Then a fanatical religious leader shows up with four children to serve as his figureheads, and does away with all her achievements.
I encourage you to add your own attempts in the comments.